The Inspection Methods of Printed Circuit Board

2018-11-23 17:56Writer: qyadminReading:
As a platform for numerous components and circuit signal transmission, Printed Circuit Boards (PCBs) have been regarded as such a key section of electronic information products that their quality determines quality and reliability of end products. Owing to uprising development trend towards high density and lead-free and halogen-free environmental requirement, if professional and timely inspection leaves undone, all kinds of failure issues possibly take place to PCBs such as bad wettability, crack, delamination etc.
  In order to ensure both high quality and reliability of assembled PCBs, PCB manufacturers and assemblers have to implement inspections for boards at different stages during manufacturing and assembly procedure so that surface defects will be eliminated. Furthermore, timely and professional inspections are capable of leading defects to be exposed prior to electrical test and are beneficial for data accumulation for Statistical Process Control (SPC). Wide application of Surface Mount Technology (SMT) raises higher requirement for inspection because SMT solder joints have to withstand more stress than those with the application of Plated-Through-Hole (PTH) technology. Since device leads depending on SMT have to stand more structural load, devices won't be firmly soldered onto the board without sufficient solder. Therefore, long-term electrical reliability of circuit boards with surface mount devices assembled on dramatically depend on structural integrity of solder joints, which adds definite necessity to PCBA inspection.

  Inspection Technology

  Up to now, apart from visual inspection, multiple structural inspection technologies are available with different cost, performance and defect coverage. Automatic inspection technologies include optical inspection, laser triangle measurement, X-ray inspection and X-ray lamination technology. To implement optimal process inspection, manufacturers should be aware of both advantages and disadvantages of each type of inspection method and make clear where each type performs best. In general, PCB assembly inspection technologies are classified into two types: visual inspection and automatic process inspection.

  a.Visual Inspection

  Visual inspection can be used after lots of steps during PCB assembly process and equipment for visual inspection is selected based on the positions of inspection targets. For example, after solder paste printing and device placement, inspection staff is able to find out obvious defects with naked eyes such as contaminative solder paste and missing components. The most prevalent visual inspection is capable of inspecting reflow solder joints by observing light rays reflected from ordinary prism from different angles. Generally speaking, this type of inspection can test 5 joints in just one second.
  Validity of visual inspection depends on inspection staff's capability, consistency and applicability of inspection standard. Inspectors have to be fully aware of technical requirement for each type of solder joints since each type of solder joints possibly contain as many as eight kinds of defect standards while over 6 types of solder joints are possibly available on different assembly devices. Thus, it's inapplicable for visual inspection to be used in quantitative measurement for effective structural process control. Moreover, visual inspection isn't fit for hidden solder joint inspection such as those on J-lead devices with high density package, ultramicro fine square flat devices, surface array flip chip or BGA (ball grid array) devices. Based on uniform and specific rules establishment, visual inspection is regarded as a technology with low cost and easy access, applicable for large defect inspection.

  b. Structural Process Test System (SPTS)

  Digitalization and analysis system of real-time and automatic video capture is capable of dramatically improving allowance and repeatability of visual inspection. Therefore, structural process test system depends on some form of emitting light like visible light, laser beam and X ray. All those systems acquire information through processing images to find out and measure defects concerned with solder joint quality. Similar with visual inspection, SPTS is implemented without the need to physically contact circuit board. Different from visual inspection, however, SPTS features such high repeatability and eliminates subjectivity from defect measurement.

  • Automatic/Automated Optical Inspection (AOI)

  AOI system relies on multiple light sources, programmable LED library and some cameras to shine solder joints and take shots. Under reflected light, leads and solder joints play a role as mirror reflecting majority of light while both PCBs and SMDs reflect little light. Light reflected from solder joints fails to provide the practical height data while graphics and intensity of reflected light provides information in terms of solder joint curvature. Then professional analysis will take place to determine whether solder joints are complete, whether solder is sufficient, whether bad wetting takes place. Apart from that, AOI system also inspect solder bridging and missing components or displacement before or after reflow soldering. AOI equipment runs at a speed of 30-50 joints per second and features relatively LOW cost. Nevertheless, it fails to inspect parameters of some solder joints such as weld height and solder in a joint and fails to inspect hidden solder joints such as those belonging to BGA, PGA and J-shape lead devices that are essential to reliability of soldering. In conclusion, AOI test performs BEST at inspection of ICs and gull-wing devices with pitch larger than 0.5mm.
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